1. Field of the Invention
The invention is generally related to the field of oxygen sorbent materials.
2. Related Art
Materials having the ability to absorb oxygen are useful in many industries, for example enrichment of oxygen from air or other multicomponent fluid. One useful class of crystalline materials is the so-called mixed ionic-electronic conductors (MIEC). These materials have exhibited enhanced sorption, as well as enhanced ionic and electronic conductivities, particularly their oxide-ion mobility and oxygen-(O2)-storing capacities. High-temperature non-stoichiometric defects (such as oxide-ion vacancies or interstitial oxygen sites) in the crystal-lattice allow the materials to temporarily sorb oxygen at high-temperature for oxygen enrichment of gas streams, as well as for catalytic transformations of other compounds using the oxygen that is temporarily absorbed.
Potential applications of MIEC materials have been documented: production of syngas and H2; partial oxidation of light hydrocarbons; production of pure O2 (with a view toward replacing, possibly, cryogenic distillation); production of O2-enriched gas mixtures; various environment-related solutions; and solid-oxide fuel cells and sensors (cf, for example, P. J. Gellings and H. J. M. Bouwmeester, Solid State Electrochemistry, CRC Press, Boca Raton, 1996; H. Ullman, Keramische Zeitschrift, 53 (2001) 100-107; S. J. Skinner and J. A. Kilner, materialstoday (March 2003) 30-37. Specifically, Ion-Transport-Membrane (ITM) technologies for O2 separation and syngas production are under quickly progressing development.
Technologies other than ITM, such as cyclic high-temperature sorption-desorption processes based on O2 partial-pressure swings, have gained considerable interest. Cyclic O2-recovery processes that utilize specific ceramic materials, such as K2NiF4 as sorbent/catalyst having high-temperature non-stoichiometric defects are being developed. These processes are referred to as ceramic autothermal recovery processes, or CAR processes, to be discussed further in the Description. They consist of alternately passing air and another medium (or applying vacuum) through pellets or granules of the material in a fixed-bed configuration, with cycle times of the order of minutes or less. Oxygen retained in the solid material during the air contacting step is released by decrease in O2-partial pressure via application of vacuum, steam, carbon dioxide, steam-carbon dioxide mixtures, flue-gas mixtures or other appropriate means, to generate an O2-enriched stream, which is used as feed to other systems, into which CAR could be integrated, e.g., combustion processes.
Key advantages of CAR over ITM processes are ease of material fabrication, plant design and process execution using traditional unit operations. Preliminary estimates indicate significant economic benefits compared to the traditional cryogenic air separation, due to lower energy consumption. On the other hand, cryogenic air separation plants are well understood and, despite their high operating costs, industry momentum is geared toward designing these plants cost-effectively, and this leads to reduced engineering and design costs, and inherent safety built up over time.
One of the key technical issues and risks of the CAR process for O2 enrichment relates to material development. Of the MIEC materials of interest, perovskites and perovskite-like materials have attracted attention in the past. The process-related crystal-structure oxide-ion, O2−, deficiency can be exemplified by perovskite-type oxides, which originally referred to the mineral CaTiO3. Today, “perovskite” denotes a series of oxygen-containing compounds with a unique general crystal structure, ABO3, with high-temperature O2− vacancies, denoted by the symbol δ, which obeys the general formula ABO3-δ. The “A”-site cations can be rare earth, alkaline earth, alkaline and other large cations such as Pb2+, Bi3+, or Ce4+, and the “B”-site cations can be 3d, 4d, or 5d transition-metal cations. Multiple cation-type occupations of those two sites are possible. Framework sites “A” and “B” are dodecahedral and octahedral, respectively, cf., L. G. Tejuca and J. L. Fierro, Properties and Applications of Perovskite-type Oxides, Marcel Dekker, New York, 1993.
A standard cubic high-temperature perovskite phase remains stable and reversible with regard to changes of δ within a certain range: The value δ could be as high as 0.25, but as a rule δ=0.05-0.25 (although higher values have been reported), at elevated temperature and low oxygen partial pressure, i.e., δ is a function of temperature and partial pressure of oxygen. On the other hand, perovskite stability is governed by cation radii of lattice metals in various valence states combined into a parameter “t” called “tolerance factor”, cf., Z. Shao, et al., Sep. Purif Technol., 25 (2001) 419-42. A perovskite structure can only be formed if t ranges from 0.75-1. These circumstances have set limits to the performance potential of perovskites in O2-recovery and related processes. Tereoka et al. described the material La1-xSrxCo1-yFeyO3-δ as a medium for oxide-ion permeation with excellent performance at high temperature (Y. Tereoka et al., Chemistry Letters, (1985) 1367-1370; 1743-1746). In this case, the O2 permeation is driven by the O2-partial-pressure difference between the two sides of the membrane, and it results from oxygen-ion transport through the vacant sites in the lattice structure. Significant efforts have been directed to the investigation of the fabrication and utilization of these materials as O2-transport membranes for gas separation and reaction. Mazanec, et al., (U.S. Pat. No. 4,933,054) were awarded a patent in this area in 1990. One recent study of SrCo0.4Fe0.6O3-δ reported, however, a decrease in oxygen permeability through samples that were purposely made stronger by addition of up to 9 wt % ZrO2, where the Zr4+ cations replaced some of the B site cations. Yang, et al., Effect of the Size and Amount of ZrO2 Addition on Properties of SrCo0.4Fe0.6O3-δ, AIChE Journal, Vol. 49, Issue 9, pages 2374-2382 (2003).
Desorption of O2 from La1-xSrxCoO3-δ was studied and reported by Nakamura et al. (Chemistry Letters, (1981) 1587-1581) in 1981. Later, Teraoka et al. (Chemistry Letters, (1985) 1367-1370) examined O2 sorption properties of La1-xSrxCo1-yFeyO3-δ. They observed that considerable amount of O2 was desorbed from this class of oxides as temperature was increased from 300° C. to 1100° C., and was absorbed as temperature was decreased. Mizusaki et al. (J. Solid State Chemistry, 80 (1989) 102) measured the oxygen non-stoichiometry of the perovskite-type oxide La1-xSrxCoO3-δ as function of temperature, Sr content (x) and O2-partial pressure. Lankhorst and Bouwmeester (J. Electrochem. Soc., 144 (1997) 1268) measured the oxygen non-stoichiometry of La0.8Sr0.2CoO3-δ. Zeng and Lin (Solid State Ionics, 110 (1998) 209-221) investigated O2-sorption and desorption rates of a La0.2Sr0.8CoO3-δ sample subjected to sudden changes of O2-Partial pressure at constant temperatures. They found that this rate could be correlated to a linear-driving force of the deviation of oxygen-vacancy concentration in the bulk phase of the sample from its corresponding thermodynamically equilibrated one.
Few patents have been issued for processes using the O2-sorption properties of perovskite-type oxides for gas separation and purification. Doi et al., KoKai Patent No. Hei 5 (1993)-4044, 1993) disclose using a perovskite-type oxide, ABO3-δ, as high-temperature O2 sorbent to remove O2-containing impurities, such as NOx by means of a TSA technique to regenerate the sorbent. A Chinese patent application by Yang, et al., Appl. No. 99 1 13004.9, describes a material, Ba0.5Sr0.5CO0.8Fe0.2O3-δ, with very high concentration of oxygen vacancies. The CAR concept, developed by Lin, et al., U.S. Pat. No. 6,059,858, also uses perovskite-type oxides as sorbents to separate O2 from an O2-containing stream, particularly air, by a type of mixed TSA-PSA process. This patent also discloses methods of sorbent regeneration using CO2 or steam as the purge gas.
U.S. Pat. No. 6,143,203 describes CAR technology extended to the area of hydrogen and synthesis-gas production using perovskite-type oxide sorbents. See also U.S. Pat. Nos. 6,379,586 and 6,464,955. A material patent of common assignment hereto has also been applied for: Supported Perovskite-Type Oxides and Methods for Preparation Thereof by Zeng, et al., U.S. Pub. Pat. Appl., U.S. 2002/0179887 A1 (2002).
As mentioned, the ability of perovskite and perovskite-like materials to function as commercial sorbents has limits. There have been recent efforts to improve the O2-sorption performance of these materials, but with limited success. U.S. Pat. No. 6,772,501 discloses composites of metals and ion conductors; U.S. Pat. No. 6,740,441 discloses using perovskites to thin-film coat “current collects” (metal screens or mesh) in solid oxide fuel cells, and other devices, and gas separations are mentioned; U.S. Pat. Nos. 6,641,626 and 6,471,921 disclose MIEC conducting membranes for HC processing, and disclose ceramic membranes which have good ionic and electrical conductivity, plus excellent strength under reactor operating conditions. The compositions comprise a matrix of MIEC (especially brownmillerite) with one or more second crystal phase, non-conductive, which enhances strength; U.S. Pat. No. 6,541,159 discloses O2-separation membranes having an array of interconnecting pores and an OH− ion conductor extending through the pores, and an electrical conductor extending through the pores, discrete from the OH− ion conductor; U.S. Pat. No. 6,440,283 discusses forming pellets, powders, and two layer structures of La/Sr oxides; U.S. Pat. No. 6,146,549 discloses La/Sr ceramic membranes for catalytic membrane reactors with high ionic conduction and low thermal expansion; U.S. Pat. Nos. 5,509,189 and 5,403,461 discuss solid solutions of pyrochlore crystal phase and perovskite crystal structures.
Despite improvements in the art, the need remains for compositions, which take better advantage of the excellent O2-sorption and permeability properties of perovskites and perovskite-like materials, while exhibiting enhanced durability, so that the materials may be used commercially.